1. Field of the Invention
The present invention relates to an oscillator using a negative resistance device, and more particularly, to an electromagnetic-wave oscillator for a frequency band in a range of, for example, from a millimeter band to a terahertz band (30 GHz or higher and 30 THz or lower; the same applies hereinafter).
2. Description of the Related Art
As is widely recognized, a negative resistance device, together with a resonator, is useful in application fields of oscillators for electromagnetic waves. It has heretofore been known to generate an electromagnetic wave containing at least a part of frequency components in a frequency band in a range of from a millimeter band to a terahertz band (hereinafter also simply referred to as “terahertz wave and the like”). As an example, IEEE Electron Device Letters, Vol 18, 218 (1997) discloses an oscillator in which a negative resistance device is monolithically integrated on a substrate. A slot antenna is integrated on the semiconductor substrate having the negative resistance device, and a resonator structure and a gain medium are monolithically formed.
FIG. 6 is a diagram illustrating the oscillator disclosed in this document. This oscillator uses, as a negative resistance device, a resonant tunneling diode S-RTD 11 provided with a Schottky barrier on the collector side. As a resonator, a slot antenna is used. The slot antenna disclosed in this document is formed of a metal pattern 12 on a semiconductor substrate, and it includes capacitors 13 and 14 at both ends of the slot antenna. The oscillator disclosed in this document further includes a rectifier diode 15. The rectifier diode 15 forms a stabilizing circuit for suppressing parasitic oscillation, which can be problematic in an oscillator using a negative resistance device. Parasitic oscillation refers to oscillation that is parasitically generated in a frequency band on the low frequency side different from a desired frequency. Such parasitic oscillation greatly decreases an oscillation output at a desired frequency, and it is therefore very important to provide the stabilizing circuit in the oscillator using the negative resistance device, as described in detail below. When the oscillator has an oscillation wavelength λosc and an oscillation frequency ωosc, in order to suppress parasitic oscillation, the power source for supplying bias needs to be low in impedance in a frequency region of DC or higher and lower than ωosc. One possible solution is to dispose a low-impedance circuit (such as a shunt rectifier diode) at a position within λosc/4 from the S-RTD on the power source side. Therefore, in FIG. 6, the rectifier diode 15 is integrated as a low-impedance circuit at a position within λosc/4 from the S-RTD 11 on a power source 16 side. This oscillator includes a resistor 17 as the sum of an internal resistance of the power source 16 and a resistance of connection lines.
However, IEEE Electron Device Letters, Vol 18, 218 (1997) merely describes a circuit configuration of the oscillator including the slot resonator using the slot antenna. This circuit configuration could not be applied to a microstrip resonator, such as a patch antenna. Specifically, the position within λosc/4 from the negative resistance device such as an RTD on the power source side corresponds to a region of the patch itself or a region that is more affected by a resonance magnetic field in the periphery of the patch. Thus, it has not been easy to dispose a low-impedance circuit.